There are some internal combustion engines which are provided with an idle speed control apparatus for controlling idle speed. The idle speed control apparatus includes a bypass inlet air passage for communicating with a main inlet air passage and bypassing an inlet throttle valve therein, and a bypass inlet air quantity control valve disposed in this bypass inlet air passage. The idle speed of the internal combustion engine is feedback controlled to a target idle speed by this bypass inlet control valve. The idle speed control apparatus also includes an auxiliary bypass inlet air passage for communicating with said bypass inlet air passage and bypassing said bypass inlet control valve. An auxiliary bypass inlet air quantity adjusting apparatus is disposed in this auxiliary bypass inlet air passage, with said target idle speed being regulated by this auxiliary bypass inlet control apparatus.
As examples of such idle speed control apparatus, there is one in which the idle speed is maintained constant by presetting an air quantity corresponding to a particular electric load into an arithmetic sequence unit and when the particular load is on or off, controlling an air bypass shutter means of the throttle valve so that the preset air quantity can be obtained (Japanese Patent Early Laid-open Publication No. Sho 60-93164). Another apparatus includes inlet air and fuel bypass passages communicating with the downstream side of a throttle valve from atmosphere and a fuel tank respectively, these bypass passages being interlockingly opened and closed to control the idle speed (Japanese Patent Early Laid-open Publication No. Sho 60-98157).
There are two cases for regulating the target idle speed by the auxiliary bypass inlet control apparatus when a maintenance inspection or repair work, etc., is to be performed. In one case, the auxiliary bypass inlet air quantity is regulated to regulate the target idle speed while the feedback control of the bypass inlet control valve is stopped to allow a fixed control. In the other case, the auxiliary bypass inlet air quantity is regulated to regulate the target idle speed while feedback control is performed by the bypass inlet control valve.
However, there is a case where even when the auxiliary bypass inlet air quantity is regulated by the auxiliary bypass inlet control apparatus while feedback control is performed by the bypass inlet control valve, the target idle speed cannot be regulated correctly. This occurs because when the bypass inlet control valve performs feedback control, if the sensitivity of the control is made sharp, hunting takes place and in order to prevent this, a certain range or blind section of engine speeds is established and extends from the overspeed side of the target idle speed to the underspeed side thereof. This blind section is also provided to prevent hunting under normal engine operational conditions, such as when the engine returns to idle from a high speed condition.
For example, as shown in FIG. 6, when an engine speed blind section A.sub.1 of .+-.50rpm is provided with respect to a target idle speed of 800rpm, if the idle speed is within the engine speed blind section A.sub.1 of 100rpm, feedback control is not performed by the bypass inlet control valve even if the auxiliary bypass inlet air quantity is regulated by the auxiliary bypass control apparatus. As a result, a correct regulation to the target idle speed is unobtainable.
Therefore, the object of the present invention is to provide an idle speed control apparatus in which when the target idle speed is regulated by the auxiliary bypass inlet control apparatus, the target idle speed can be regulated correctly while feedback control is being performed by the bypass inlet control valve.